1. Field of the Invention
The present invention relates generally to an apparatus accommodating a rotor and a rotor shaft secured to each other, and more particularly to an improved rotor shaft for mounting a rotor of light alloy thereon and a method of making the rotor shaft.
2. Description of the Prior Art
Japanese Utility Model Laid-open Application (unexamined) No. 1-93338 discloses a mechanical super charger employing a screw compressor.
In this kind of super charger, two rotors are secured to respective rotor shafts and a pair of timing gears in mesh with each other are secured to the drive ends of the rotor shafts, respectively. Furthermore, a pulley is secured to one of the rotor shafts and is driven by a crank shaft of an engine for compression of intake air.
In this case, the rotor shafts are generally made of steel whereas the rotors are generally made of light alloy, for example aluminum alloy, to resist high speed rotation and to aim at weight-saving.
However, since aluminum alloy is greater in the thermal coefficient of expansion than steel, a unit accommodating a rotor of aluminum alloy securely mounted on a rotor shaft of steel is disadvantageous in that the difference in thermal expansion causes looseness of the rotor when they become warm, thus reducing the gripping force of the rotor with respect to the rotor shaft. As a result, there occasionally arises a problem in that seizing occurs on the rotor or the compressive force reduces.
In order to solve this problem, it is considered to make the thermal coefficient of expansion of the rotor shaft approximate to that of aluminum alloy for prevention of the looseness between the rotor and the rotor shaft. To this end, the rotor shaft is designed to have a high thermal coefficient of expansion by the use of steel containing the austenitic structure (for example, austempered steel, austenitic stainless steel, or the like).
However, bearings of steel carrying the rotor shaft cannot absorb large thermal expansion of the rotor shaft and the difference in thermal expansion between the rotor shaft and the bearings occasionally causes plastic deformation in the bearings when they become warm.
In the case of a rotor shaft of steel containing the austenitic structure, austenite transforms to martensite and the rotor shaft becomes hard during machining, thereby lowering the machinability thereof, for example, when thread cutting is performed with respect to an end or ends thereof.